This invention relates to repeaters used to transmit information from one transmission medium to another transmission medium.
Repeaters are widely used in the transmission of information throughout a system, and are particularly useful in the transmission of information throughout a local area network (LAN). For example, repeaters are used to connect coaxial cable segments together to form a longer transmission medium since there are strict limits on the allowable length of any one segment. FIG. 1 shows a station 10, being connected to a station 22 through three cable segments 12, 14, 16 and two repeaters 18, 20. According to IEEE 802.3 standards for LANS, a segment may not be longer than 500 meters. Therefore repeaters 18, 20 in FIG. 1 are used to connect stations up to 1,500 meters apart.
FIG. 2 illustrates the connection of a plurality of stations to a LAN cable segment. Each of stations 30 (numbered 1 to N) is connected to segment 32 which is connected to LAN segment 34 through repeater 36. Therefore, repeater 36 provides access for each station 30 to the LAN segment 34.
A repeater transmits information by receiving the information on one cable segment and repeating the received information onto the other cable segment to which it is attached. The repeater may perform a variety of signal processing operations on the received data before it is repeated, such as signal amplification, signal retiming, preamble insertion, etc.
Since each of stations 30 utilize the same medium to transmit information (i.e., segments 32 and 34), only one station can be transmitting information at a given time. Accordingly, before a station transmits information to a segment it will "listen" to the segment to determine if the segment is "available". The station will transmit information only to a segment which is available. Nevertheless, a "collision" may still occur since it is possible that both will test the desired cable segment simultaneously and, since both would hear a quiet cable segment, both may simultaneously begin transmitting data. Repeaters automatically repeat whatever data they receive, and, therefore, a collision on one segment will quickly spread to the entire network. Therefore, stations are able to detect such collisions. When collisions occur, a station will typically wait a predetermined period of time and again attempt transmission.
The reliability of the operation of a LAN depends a great deal on the reliability of the repeaters in the system. For example, in the system of FIG. 1, if one repeater fails, communication between station 10 and station 22 will be impossible. In a more elaborate network, using more repeaters, the likelihood that one will fail and therefore the likelihood that the system will fail, increases. In a system such as the one illustrated in FIG. 2, the failure of repeater 36 will terminate all communication between each station 30 and LAN segment 34. When such a failure occurs, the time required to isolate and replace the failed repeater can be significant.
In order to provide a more secure sytem, i.e., one that won't fail with the failure of a single repeater, bridges were developed to connect transmission mediums. A typical system utilizing bridges instead of repeaters is illustrated in FIG. 3. Bridges 40, 42 connect cable segments 44, 46 to enable stations 48 (numbered 1 to N) to communicate with stations (not shown) on LAN segment 44.
Bridges 40, 42 are capable of receiving information on either of the cable segments and transmitting the information to the other segment (much like a repeater). Information is transmitted in packets of a fixed duration and, for example, a packet transmitted on segment 46 by one of stations 48 will be received by each of bridges 40, 42. It can be clearly seen that if bridges 40, 42 acted like standard repeaters (or, if standard repeaters were connected in a manner similar to the parallel connection of bridges 40, 42), each would attempt to transmit the received packet onto cable segment 44, resulting in a collision. This would happen every time information was received over either segment 46 or segment 44, rendering the system inoperable.
However, bridges 40, 42 are software controlled devices (unlike hardware controlled repeaters) which commuicate with one another before tranmsitting received infomation. When a packet is received by the bridges, one of the bridges (designated in advance) will "tell" the other bridge not to transmit the received packet and will then proceed to transmit the packet itself, thereby avoiding a collision. Bridges also "act" like stations in that they will listen to a segment before transmitting to be sure that it is available.
If one of the bridges should fail the other can assume all of the packet transmission functions until the failed bridge is replaced. This will allow the system to continue to operate in the event of a failure and each of the stations will continue to have access to the LAN.
Bridges may be similarly used in a system such as that illustrated in FIG. 1 and can prevent a break in the system due to failure.